In many building a drop ceiling is spaced below a structural floor panel that is constructed of concrete, for example. Light fixtures as well as other items appear below the drop ceiling. The space between the ceiling and the structural floor panel from which it is suspended serves as a return-air plenum for elements of heating and cooling systems as well as a convenient location for the installation of communications cables including those for computers and alarm systems. It is not uncommon for these plenums to be continuous throughout the length and width of each floor. Also, the space under a raised floor in a computer room is considered a plenum if it is connected to a duct or to a plenum.
When a fire occurs in an area between a floor and a drop ceiling, it may be contained by walls and other building elements which enclose that area. However, if and when the fire reaches the plenum, and if flammable material occupies the plenum, the fire can spread quickly throughout an entire story of the building. The fire could travel along the length of cables which are installed in the plenum if the cables are not rated for plenum use. Also, smoke can be conveyed through the plenum to adjacent areas and to other stories.
A non-plenum rate cable sheath system which encloses a core of insulated copper conductors and which comprises only a conventional plastic jacket may not exhibit acceptable flame spread and smoke evolution properties. As the temperature in such a cable rises, charring of the jacket material may occur. Afterwards, conductor insulation inside the jacket begins to decompose and may char. If the jacket char retains its integrity, it functions to insulate the core; if not, it ruptures either by the expanding insulation char, or by the pressure of gases generated from the insulation exposed to elevated temperature, exposing the virgin interior of the jacket and insulation to elevated temperatures. The jacket and the insulation begin to pyrolize and emit more flammable gases. These gases ignite and, because of air drafts with the plenum, burn beyond the area of flame impingement, propagating flame and generating smoke and possibly toxic and corrosive gases.
As a general rule, the National Electrical Code (NEC) requires that power-limited cables in plenums be enclosed in metal conduits. The initial cost of metal conduits for communications cables in plenums is relatively expensive. Also, conduit is relatively inflexible and difficult to maneuver in plenums. However, the NEC permits certain exceptions to this requirement provided that such cables are tested and approved by an independent testing agent such as the Underwriters Laboratories (UL) as having suitably low flame spread and smoke-producing characteristics. The flame spread and smoke production of cable are measured using UL 910, Standard Test Method for Fire and Smoke characteristics of Electrical and Optical-Fiber Cables Used in Air-Handling Spaces. See S. Kaufman "The 1987 National Electric Code Requirements for Cable" which appeared in the 1986 International Wire and Cable Symposium Proceedings beginning at page 545.
The prior art has addressed the problem of cable jackets that contribute to flame spread and smoke evolution also through the use of fluoropolymers. These together with layers of other material, have been used to control char development, jacket integrity and air permeability to minimize restrictions on choices of materials for insulation within the core. Commercially available fluorine-containing polymer materials have been accepted as the primary insulative covering for conductors and as a jacketing material for plenum cable without the use of metal conduit. In one prior art small size plenum cable, disclosed in U.S. Pat. No. 4,605,818 which issued on Aug. 12, 1986 in the names of C. J. Arroyo, et al., a sheath system includes a layer of a woven material which is impregnated with a fluorocarbon resin and which encloses a core. The woven layer has an air permeability which is sufficiently low to minimize gaseous flow through the woven layer and to delay heat transfer to the core. An outer jacket of an extrudable fluoropolymer material encloses the layer of woven material. In the last-described cable design, a substantial quantity of fluorine, which is a halogen, is used. Fluoropolymer materials are somewhat difficult to process especially for insulation covers. Also, some of those fluorine-containing materials have a relatively high dielectric constant which makes them unattractive as insulation for communications conductors.
Further, a fluoropolymer is a halogenated material. Although there exist cables which include halogen materials and which have passed the UL 910 test requirements, there has been a desire to overcome some problems which still exist with respect to the use of halogenated materials such as fluoropolymers and polyvinyl chloride (PVC). These materials exhibit undesired levels of corrosion. If a fluoropolymer is used, hydrogen fluoride forms under the influence of heat, causing corrosion and a level of toxicity which is not as low as desired.
Generally, there are a number of halogenated materials which pass the industry tests. However, if halogenated materials exhibit some less than desired characteristics as required by industry standards in the United States, it is logical to inquire as to why non-halogenated materials have not been used for cable materials. The prior art has treated non-halogenated materials as unacceptable because, as a general rule, they are not as flame retardant or because they are too inflexible if they are flame retardant. Materials for use in communications cables must be such that the resulting cable passes as industry standard flame test. For example, for plenum cable, such a test is the UL 910 test. The UL 910 test is conducted in an apparatus which is known as the Steiner Tunnel, Many non-halogenated plastic materials have not passed this test.
Non-halogenated materials have been used in countries outside the United States. One example of a non-halogenated material that has been offered as a material for insulating conductors is a polyphenylene oxide plastic material. Inasmuch as this material has not passed successfully industry standard tests in the United States for plenum use, there have been ongoing efforts to provide a non-halogenated material which has a broad range of acceptable properties, as well as a reasonable price and yet one which passes the UL 910 test for plenum cables. Such a cable should be one which appeals to a broad spectrum of customers.
In recently filed U.S. application Ser. No. 303,212 (filed Jan. 27, 1989), pending, a plenum cable is disclosed in which each transmission medium may include optical fiber or metallic conductors. Each transmission medium is enclosed with a non-halogenated plastic material selected from the group consisting of a polyetherimide, a silicon-polyimide copolymer or blends of these two materials. A jacket encloses the core and is made of a non-halogenated plastic material which includes a silicone-polyimide copolymer or its blend with a polyetherimide.
Although it is believed that the use of non-halogenated materials for both insulation and jacket in plenum cables will enjoy wide acceptance in the future, there is still a need for a plenum cable for the near term which overcomes problems of the prior art cables but which is not such a radical departure from the past. Such a cable would be one which is cost competitive with those now used and which incorporates at least some of the present technology.
The sought-after cable not only exhibits suitably low flame spread and low smoke producing characteristics provided by currently used cables which include only halogenated materials but also is one which meets a broad range of desired properties such as acceptable levels of corrosivity and toxicity. Such a cable does not appear to be available in the prior art. What is further sought is a cable which is characterized as having relatively lower corrosive and toxicity properties, as well as low levels of smoke generation when compared to those properties of existing cables and one which is readily processable at reasonable costs.